Shear strength reduction method and apparatus

ABSTRACT

A method of imposing a simple harmonic wave of bidirectional vibration on an object stuck in sand in a well bore, where vibrations are imposed in close proximity to the stuck location, and where the harmonic wave has sufficient amplitude and frequency to create soil liquefaction at the surface of the stuck object, thereby reducing the amount of overpull necessary to free the object from the sand.

CROSS REFERENCE TO RELATED APPLICATIONS

Not Applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is in the field of downhole jarring devices usedin oil and gas well drilling and downhole equipment recovery. Morespecifically, it is a method for loosening a tubular object stuck insand, by imparting vibration to the tubular object.

2. Background Art

In well operation, there is often a need for jarring, impact orvibration devices to move tubular objects that are stuck in a well bore,as a result of excessive friction with sand at a downhole location,often called “sticktion” or “stiction” force. The tubular object may bea work string or a production tube. It may be stuck in a sand formationin an open hole, or it may be stuck in a cased hole, where there is agravel pack or completion sand between the tubular object and thecasing. The terms “sand” and “soil” are used interchangeably herein, andother similar substances such as gravel are intended to be included. Onemethod employed to loosen such stuck objects is the use of the impactjar. These are typically included in a pipe or work string, near thedepth at which the object is stuck in the sand, to provide largeamplitude, unidirectional pulses or impacts of very short duration. Theamplitude of the pulse is typically between 6 and 8 inches, and theduration of the pulse is typically in the range of 10 to 100milliseconds. Impact jars are also usually single impact devices whichmust be recocked each time before operation, so they typically impartpulses 1 or 2 minutes apart, or in the frequency range of about 0.02 to0.03 Hz. Therefore, only a limited amount of energy can be delivered toa stuck object over a given period of time, with this type of tool. Thistype of loading does not produce a favorable rate of buildup of porepressure, in the sand in which the tubular object is stuck. Therefore,the necessary reduction of soil strength in the area surrounding thestuck portion of the object, as a result of soil liquefaction, is notrealized, and the stiction force must be overcome by a significantamount of overpull on the work string.

Some of the known impact tools require the operator to pull up on thework string with a force sufficient to pre-stress the work string,thereby providing the motive force for an impact. The impact istypically initiated when some type of valve or other triggering devicein the tool triggers an action which applies the energy stored in thepre-stressed work string in the form of an impact delivered to the stucktubular object. The force of the impact delivered by such a tool dependsupon how much energy is stored in the pre-stressed work string. That is,a larger over-pull will deliver a harder blow to the stuck portion ofthe tubular object. Because of the aforementioned limitations of thistype of tool in reducing the friction force on a tubular object stuck ata deep location, the energy put into the system in the form of overpullhas to be very large, in order to overcome the stiction force betweenthe stuck object and the sand, and in order to mechanically break theinterface bond between the tubular object and the sand.

A second method for loosening a tubular object stuck in sand is theapplication of bi-directional, simple harmonic, vibrations of asufficient amplitude and frequency to induce soil liquefaction, which inturn reduces the stiction force between the sand and the tubular object.The vibration amplitude is in the range of about 0.6 inch to 0.8 inch.Rather than being discrete pulses, the vibrations are continuouslyapplied, at a frequency of up to about 60 Hz. As compared to the use ofthe impact jar, this method requires much less overpull; in fact, therequired overpull may be only a fraction of the original stiction forceon the stuck object. Where sufficient vibration energy can be applied atthe stuck location, this method can be very effective. That is, thismethod results in a high degree of soil liquefaction and a high degreeof friction force reduction, resulting in a comparatively low extractionforce requirement. However, in this method, the vibrations are impartedto the tubular object at the Earth's surface, and the tool has a limitedability to propagate the vibrations to great depths in the well bore.So, a tubular object can not be extracted by this method, if it is stuckin sand at a greater depth than the depth to which the tool canpropagate sufficient vibration energy.

A third method employed to extract a tubular object stuck in sand isdelivering energy to the soil mass in the form of uni-directional pulsessimilar to those delivered by the impact jar, except that the pulseamplitude is much smaller, and the pulses are more closely spaced. Thepulses are delivered to the tubular object near the stuck location. Thepulse amplitude in this method is typically about 0.06 inch to 0.08inch, the pulse duration is typically about 0.003 seconds, and the pulsefrequency is typically about 10 to 20 Hz. Spacing the pulses moreclosely assists in pore pressure buildup, but the smaller pulseamplitude is generally not great enough to induce plastic strains in thesoil. As a result, the degree of soil liquefaction is only moderate, andthe resultant reduction in the stiction force is only moderate. Becauseof the location of this type of tool near the stuck depth, the depthrange of this method can be great, but the amount of extraction forcerequired can still be appreciable.

BRIEF SUMMARY OF THE INVENTION

The method of the present invention includes a bi-directionalapplication of vibration to the stuck tubular object, very near thestuck location, where the vibrations form a simple harmonic wave ofsufficient amplitude to induce soil liquefaction at the stuck location.The method may be performed by any apparatus capable of imposing therequired bi-directional vibrations on the stuck tubular object.

The novel features of this invention, as well as the invention itself,will be best understood from the attached drawings, taken along with thefollowing description, in which similar reference characters refer tosimilar parts, and in which:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The FIGURE is a schematic of an apparatus in which the method of thepresent invention can be performed.

DETAILED DESCRIPTION OF THE INVENTION

The Figure shows a tubular assembly 10 which has become stuck in sand Sat a location downhole in a well bore WB. The assembly 10 includes atubular such as a work string 12, along with a vibratory apparatus14,18,20, attached to the stuck object or fish 16. The well bore isillustrated as being a cased hole, but it may be either open hole orcased hole, and the sand in which the fish 16 is stuck may be a sandformation, completion sand, gravel pack, or other similar substance. Thelocation at which the fish 16 is stuck is commonly referred to as thestuck point SP. The vibratory apparatus 14,18,20 which will be used toperform the method of the present invention may have been incorporatedinto the tubular assembly 10 before its initial tripping into the wellbore, or it may be lowered on the work string 12 and attached to thefish 16 after the fish becomes stuck. In either case, the vibratoryapparatus should be installed at or very near the stuck point on thefish, and the vibratory apparatus 14,18,20 is adapted to deliver itspulses at or very near the stuck point SP.

The vibratory apparatus itself, by way of example only and withoutlimtation, can include a valving arrangement 14, a cycling mass 18, anda bi-directional accelerator 20. As is known in the art, fluid can bepumped downhole through the work string 12 and through the valvingarrangement 14. Operation of the valving arrangement 14 can be used tocause the fluid flow to alternatingly load the accelerator 20 in theuphole and downhole directions, then to release the accelerator 20 toact against the cycling mass 18 and deliver vibrations to the fish 16 inalternating uphole and downhole directions.

Typically, the accelerator 20 would include one or more biasing elementssuch as springs. Other energy storing devices, such as fluidaccumulators, could be used. The cycling mass 18 could be moved by thefluid flow to load the accelerator 20 in the uphole direction, forinstance, then the accelerator 20 would be released to move the cyclingmass 18 and deliver a pulse in the downhole direction, followedimmediately by hydraulic movement of the mass 18 in the downholedirection and subsequent release and delivery of a pulse in the upholedirection. By repetition of this process, alternating pulses ofsubstantially equal magnitude are delivered by cycling the mass 18 inthe uphole and downhole directions to create bi-directional vibrations.Continuous movement of the cycling mass 18 is preferred. The energycomes downhole in the form of the fluid flow; it is repetitively storedin the accelerator 20 and released, to repetitively accelerate thecycling mass 18 in alternating directions. This imposes a bi-directionalsimple harmonic wave on the tubular assembly 10, with the vibrationsbeing applied at or very near the stuck point SP on the fish 16. Othermechanisms for generating excitations in alternating directions couldalso be used, such as the directing of fluid in alternating directions.

Regardless of the type of vibratory apparatus used, the frequency of thevibratory tool can be tuned to match the natural or fundamentalfrequency of the tubular assembly 10, in order to set up a simpleharmonic wave in the tubular assembly 10. Alternatively, the frequencyof the vibratory tool can be tuned to match a whole number multiple ofthe fundamental frequency of the tubular assembly 10. The amplitude ofthe wave, the amount of cycling mass 18, and the magnitude of the energyrepetitively stored and released by the accelerator 20, are selected tointroduce sufficient energy into the tubular assembly 10 and thesurrounding sand S to generate soil liquefaction at the interfacebetween the fish 16 and the sand S. Attachment of the vibratoryapparatus at or very near the fish 16 limits the attenuation of thevibratory energy by the tubular assembly 10 itself, and insures theapplication of the greatest possible fraction of this energy at thefish/soil interface. When soil liquefaction is induced, the amount ofoverpull necessary to pull the fish 16 free from the sand S is greatlyreduced.

Because the size, thickness, shape, and materials of the tubularassembly 10 will vary greatly from one application to another, thefrequency and amplitude of the vibration will necessarily be varied.However, the vibration must always be bi-directional, it must always bea simple harmonic wave, and it must always be applied at or very nearthe stuck point SP. In typical types and sizes of tubulars used in oiland gas well drilling and production, excitation amplitudes of at leastone 1 inch are anticipated, at frequencies in the range of 10 to 20hertz, with the bidirectional movement of the cycling mass beingessentially continuous.

While the particular invention as herein shown and disclosed in detailis fully capable of obtaining the objects and providing the advantageshereinbefore stated, it is to be understood that this disclosure ismerely illustrative of the presently preferred embodiments of theinvention and that no limitations are intended other than as describedin the appended claims.

1. A method for extracting an object stuck in sand, comprising:attaching a vibratory tool to a stuck object, at a well bore depth atwhich said stuck object is stuck by friction with sand; generatingcontinuous bidirectional vibration in a simple harmonic wave form withsaid vibratory tool; and imparting said continuous harmonic vibration tosaid stuck object, at said well bore depth at which said stuck object isstuck.
 2. The method recited in claim 1, further comprising generatingsaid continuous harmonic vibration with a sufficient amplitude to inducesoil liquefaction in said sand.
 3. The method recited in claim 2,wherein said continuous harmonic vibration is generated with anamplitude of at least 1 inch.
 4. The method recited in claim 1, furthercomprising generating said continuous harmonic vibration with asufficient frequency to impart mechanical energy at a rate sufficient toinduce soil liquefaction in said sand.
 5. The method recited in claim 4,wherein said continuous harmonic vibration is generated with a frequencybetween about 10 Hz and about 20 Hz.
 6. The method recited in claim 1,further comprising generating said continuous harmonic vibration at thefundamental frequency of said stuck object.
 7. The method recited inclaim 1, further comprising generating said continuous harmonicvibration at a whole number multiple of the fundamental frequency ofsaid stuck object.
 8. A method for extracting an object stuck in sand ina well bore, comprising: attaching a vibratory tool to a stuck object ata well bore depth at which said stuck object is stuck by friction withsand; generating continuous bidirectional vibration in a simple harmonicwave form with said vibratory tool, said continuous harmonic vibrationhaving sufficient amplitude and frequency to induce soil liquefaction insaid sand; and imparting said continuous harmonic vibration to saidstuck object, at said well bore depth at which said stuck object isstuck.
 9. The method recited in claim 8, wherein said continuousharmonic vibration is generated with an amplitude of at least 1 inch.10. The method recited in claim 8, wherein said continuous harmonicvibration is generated with a frequency between about 10 Hz and about 20Hz.
 11. The method recited in claim 8, further comprising generatingsaid continuous harmonic vibration at the fundamental frequency of saidstuck object.
 12. The method recited in claim 8, further comprisinggenerating said continuous harmonic vibration at a whole number multipleof the fundamental frequency of said stuck object.
 13. An apparatus forextracting an object stuck in sand in a well bore, comprising: avibratory tool adapted to generate continuous bidirectional vibration ina simple harmonic wave form; a mechanical connection adapted to attachsaid vibratory tool to a stuck object substantially at a well bore depthat which said stuck object is stuck by friction with sand, saidconnection being adapted to transmit said continuous harmonic vibrationto said stuck object, substantially at said well bore depth at whichsaid stuck object is stuck; and a work string adapted to position saidvibratory tool substantially at said well bore depth at which said stuckobject is stuck.
 14. The apparatus recited in claim 13, wherein saidvibratory tool is further adapted to generate said continuous harmonicvibration with a sufficient amplitude to induce soil liquefaction insaid sand.
 15. The apparatus recited in claim 14, wherein said vibratorytool is further adapted to generate said continuous harmonic vibrationwith an amplitude of at least 1 inch.
 16. The apparatus recited in claim13, wherein said vibratory tool is further adapted to generate saidcontinuous harmonic vibration with a sufficient frequency to impartmechanical energy at a rate sufficient to induce soil liquefaction insaid sand.
 17. The apparatus recited in claim 16, wherein said vibratorytool is further adapted to generate said continuous harmonic vibrationwith a frequency between about 10 Hz and about 20 Hz.
 18. The apparatusrecited in claim 13, wherein said vibratory tool is further adapted togenerate said continuous harmonic vibration at the fundamental frequencyof said stuck object.
 19. The apparatus recited in claim 13, whereinsaid vibratory tool is further adapted to generate said continuousharmonic vibration at a whole number multiple of the fundamentalfrequency of said stuck object.